Thiophene dioxides reactions

Figure 8. Catalytic cycles for the Diels-Alder reactions of benzoquinone 47 with cyclohexadiene 48 (top) and tetramethyl thiophene dioxide 51. While the reaction of 47 with 48 suffers from product inhibition, adduct 52 is easily replaced by new reactants inside the capsule.

When thiophene dioxide (106) was used as the diene component, true catalysis was observed with 107, affording the capsule bound adduct 108 (equation 32)93. The displacement of a single molecule of adduct by two molecules of starting material is, in principle, disfavored on entropic grounds, but turnover took place in this case due to the poorer affinity of the Diels-Alder adduct for the capsule. The rate enhancement of this reaction, based on the ratio of the half-life for the reaction outside vs inside the capsule, was 10-fold. 

The organization of material in this chapter follows closely that set down in Chapter 3.02. The only points of departure are in the inclusion of a small section on photosubstitution (3.14.2.11) and in the bifurcation of the section on the oxides of thiophene the reactions of thiophene 1-oxides are described in Section 3.14.4.1 and those of thiophene 1,1-dioxides in Section 3.14.4.2. 

Diels-Alder reactions of thiophene 1,1-dioxides are very common (B-67MI31401). With rare exceptions, the adduct loses a molecule of SO2 to form a diene which may add another molecule of the dienophile. The sequence with maleic anhydride is shown in Scheme 193. It has been claimed that in the reaction of thiophene dioxide with acetylenedicarboxylic ester the primary adduct could be isolated, but this could not be substantiated (75RTC85). 

The normal reactions of benzo[6]thiophene 1,1-dioxides have been reviewed (70AHC(11)177). Electrophilic substitution (nitration, bromination) takes place at position 6. 3-Halo derivatives undergo normal nucleophilic displacement reactions, but 2-bromobenzo[6]thiophene dioxide gives the 3-ethoxy derivative in ethanolic NaOH. The reaction of 3-methoxy derivatives with secondary amines can give rise either to enamines... 

Cyclopropene also adds to less reactive, acyclic, dienes218), though it is worth noting that the reaction with cyclohexadiene only proceeds in 10% yield 219). Addition of a range of alkylcyclopropenes to thiophene dioxides leads to cycloheptatrienes, presumably by cheletropic elimination of sulphur dioxide from the intermediate adduct 220) ... 

This has been essentially accomplished in a case of the diene-dienophile monomer polymerization (6, 7, 8, 16). This excellent work makes use of the reaction of such dienes as cyclopentadienones, a-pyrones, and thiophene dioxides with dienophiles. The resulting adduct, a protected or inactive diene, loses carbon monoxide, carbon dioxide, or sulfur dioxide at elevated temperatures to form a diene. Thus when a Ws-malei-mide reacts with one mole of a cyclopentadienone, the resulting adduct will lose carbon monoxide at 150—260° in an inert solvent to form an active intermediate diene-dienophilic monomer which readily poly-... 

In the case of thiophene dioxide, dimerization is preferred to the reaction with a good dienophile such as maleic anhydride and thus the monomer is not suitable for a copolymerization reaction of this type. Certain substituted thiophene dioxides, however, react more readily with good dienophiles and dimerization does not become important in their polymerization reactions (6,16). Thus with a series of 6is-maleimides, 2,5-dimethylthiophene-1,1-dioxide, 3,4-diphenylthiophene-l,l -dioxide and 3,4-dichlorothiophene-l,1-dioxide readily form high molecular weight polymers. 

The most successful polymerizations carried out by using a Diels-Alder step-growth reaction are those which generate a highly reactive A-B monomer in situ by the reaction of a bismaleimide with cyclopenta-dienone (12), 2-pyrone (6, 13), or thiophene dioxide (5) derivatives. The intermediate 1 1 adduct loses carbon monoxide, carbon dioxide, or sulfur dioxide, respectively, all to generate the same type of reactive AB monomer, which is converted rapidly to polymer. High molecular weight polymers are obtained (Reaction 4). 

The primary adducts, cyclohexadiene derivatives, formed by [4+2] cycloaddition of thiophene dioxides with dienophiles, may further undergo [4+2] cycloaddition with the dienophiles. Thus, the adducts 84 of 3,4-di-ferf-butylthiophene dioxide 83 with maleic anhydride and AT-phenylmaleimide further react with these dienophiles to give excellent yields of bis-adducts, which are composed of the endo-endo and endo-exo isomers, 85a and 85b (Scheme 49) [160]. A similar reaction was also observed with 3,4-dichlorothiophene 1,1-dioxide with N-butyl- and A-p-nitrophenylmaleimides (Scheme 50) [133]. The reaction of highly congested thiophene dioxides 87 with 4-phenyl-1,2,4-triazoline-3,5-dione provides a unique pyridazine synthesis since the bis-adducts 88 are converted into the corresponding pyridazines 89 in one pot and in good yields by treatment with KOH in methanol (Scheme 51) [174]. 

The synthetically most important reaction that belongs to this category would be [4+2] cycloadditions of highly congested thiophene dioxides with alkynic dienophiles (Scheme 55) [39,40,159]. These reactions afford o-di-ferf-butyl-, o-dineopentyl-, and o-di(l-adamantyl)benzene derivatives in good yields, synthesis of which is very difficult by other means. 

Only one example of an ene reaction is reported [40]. As already described, ben-zyne and thiophene dioxides undergo [4+2] cycloaddition. However, the reaction of 3,4-dineopentylthiophene 1,1-dioxide with benzyne affords compound 98, the product of an ene reaction. The other two products, 99 and 100, result from [4+2] cycloaddition (Scheme 59). 

Most of the synthetic approaches towards benzoannelated [2.2]paracyclo-phanes use the Diels-Alder reaction to build up the annelated rings [18,22,29]. [2.2]Paracyclophane-l-ene (32) and [2.2]paracyclophane-1,9-diene (42) can be considered as the simplest dienophiles for this purpose. Since the first syntheses of these compounds in 1958 by Cram et al. [32], various attempts have been made to get them to react with dienes in terms of Diels-Alder reactions. However, [2 + 4] cycloadditions were never observed [33], and could not be facilitated either by the application of high pressure, or the presence of Lewis acid catalysts Diels-Alder adducts were not even obtained with dienes such as tetrachloro-thiophene dioxide, known for its high reactivity in [2 + 4] cycloadditions with an inverse electron demand. All the more surprising was the observation that monoene 32 reacts smoothly with dimethyl l,2,4,5-tetrazine-3,6-dicarboxylate at room temperature leading to a dihydropyridazine-annelated paracyclophane 39 in high yield [34a]. As reported for other tetrazine Diels-Alder reactions... 

Reaction at Sulphur Thiophen Dioxides. - The stable S-ylide (167) was formed, in 40% yield, in the reaction of 2-isobutenylthiophen with dimethyl diazomalonate in the presence of rhodium acetate.The transient thiophen... 

Thiophen Dioxides and their Dihydro and Tetrahvdro Derivatives. - Flash vacuum pyrolysis of cobalt complexes (194) of thiophen 1 1-dioxides led to extrusion of sulphur dioxide and the formation of cyclobutadiene complexes (195). Stereochemical labelling experiments were employed in order to determine the mechanism of this transformation.114 A detailed publication on the reaction of... 

Dimethoxythiophene (102) treated with PTAD in methanol affords a high yield of 103, a product of the Diels-Alder reaction and subsequent solvolysis with methanol [82JCS(CC)1033. Reaction of thiophene dioxides 104 bearing bulky substituents at positions 3 and 4 with excess of PTAD gives good yields of 105, products of bis addition and extrusion of sulfur dioxide. Treatment of these adducts with potassium hydroxide at room... 

Although the reaction of arynes with thiophenes as a route to naphthalenes leaves much to be desired, the reaction with thiophene-1,1 -dioxides is synthetically useful. An excess of benzyne precursor (benzenediazonium carboxylate) is used, and the solvent was refluxing 1,2-dichloroethane. Yields of naphthalenes are moderate and conversions are good. Thus 3-methylbenzyne and tetrachlorothiophene dioxide 296 gives naphthalene 297 in 65% yield (96% conversion). The thiophene dioxides may be prepared by m-CPBA oxidation of the corresponding thiophenes. 

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